Thermosetting resins such as epoxy resins, vinyl ester resins, unsaturated polyester resins, phenolic resins, bismaleimide resins, and cyanate resins are used in a wide range of applications in various fields because of their excellent features such as heat resistance, mechanical strength, and dimensional accuracy. In particular, epoxy resins, which are excellent in many respects including mechanical strength, electrical insulation properties, heat resistance, and adhesion, are used in various applications including construction materials, electrical and electronic materials, adhesives, and fiber reinforced composite materials. However, cured products thereof have low fracture toughness, and may show extreme brittleness. Their brittleness is problematic in various applications. In addition to improvement in the toughness of resins, improvement in the impact resistance has also been demanded in recent years.
As a method for improving the toughness of an epoxy resin, a method of adding a rubber to the epoxy resin is known. As rubbers to be added, a carboxyl-terminated butadiene-acrylonitrile copolymer rubber (CTBN) and a nitrile rubber have been proposed (e.g. Patent Literature 1). These methods allow a rubber to be dissolved in an epoxy resin composition, and to remain in the epoxy resin composition after cure, and the remaining rubber unfortunately degrades the heat resistance (glass transition temperature: Tg) and elastic modulus of the epoxy resin composition after cure. Another disadvantage is that the morphology of cured products varies depending on curing conditions, and therefore the physical properties are variable.
In order to solve this problem, another proposed method is to use rubbery polymer microparticles that are previously prepared as particles in an aqueous medium by a polymerization technique such as emulsion polymerization or suspension polymerization, and are substantially insoluble in an epoxy resin composition (e.g. Patent Literatures 2 and 3). There are some advantages of this method that the rubber component is previously crosslinked to render it insoluble in the epoxy resin composition, and therefore is less likely to degrade the heat resistance of the resin, and the blending and curing conditions are less likely to affect the state of dispersion. Yet, there remains the problem of essentially degrading the elastic modulus of the resin which is caused by the addition of the rubber component.
In this context, still another proposed method is to add crosslinked polymer microparticles that are free of rubber components to an epoxy resin composition (e.g. prior art document 1). This method improves the fracture toughness almost or completely without degrading the elastic modulus and heat resistance. A disadvantage of this strategy is that addition of such crosslinked polymer microparticles does not improve the impact resistance of the resin.